Heat-developable photosensitive materials have been already proposed from of old. For instance, such materials are disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075, and B. Shely, Thermally Processed Silver Systems in “Imaging Processes and Materials”, Neblette's 8th ed., p. 2, compiled by Sturge, V. Walworth & A, Shepp (1996).
In general, a heat-developable photosensitive material has a photosensitive layer containing a catalytic amount of photocatalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., an organic silver salt) and a toning agent for controlling tone of silver, dispersed in a binder matrix. After imagewise exposure, the heat-developable photosensitive material is heated at a high temperature (e.g., at least 80° C.) to cause a redox reaction between the silver halide or reducible silver salt (functioning as an oxidizing agent) and the reducing agent, thereby forming black silver images. The redox reaction is accelerated by the catalytic action of latent images formed from the silver halide by exposure. Accordingly, the black silver images are formed in the exposed areas.
The heat-development processing requires no processing solutions in contrast to wet development processing, and has advantages in its simplicity and rapidity. However, methods of forming images by wet development processing constitute the mainstream in the field of photographic technology even now. And outstanding problems missing in the wet development processing remain in the heat-development processing.
One of the problems is discoloration of dyes. It is common practice to add dyes to a photographic light-sensitive material for the purposes of filter and preventing halation and irradiation from occurring. The dyes are added to light-insensitive layers, and function at the time of imagewise exposure. When the dyes remain in the photographic light-sensitive material after they have finished functioning, the images formed are colored by the remaining dyes. Accordingly, it is necessary to remove the dyes from the photographic light-sensitive material in development processing. In the wet development processing, the dyes can be easily removed from the photographic light-sensitive material by use of a processing solution. In heat-development processing, on the other hand, removal of the dyes is very difficult (or impossible in a practical sense).
In the recent photographic technology, especially in the technical fields of medical photography and graphic arts photography, simplicity and rapidity are demanded of development processing. However, improvements of wet development processing come up nearly to their limits. In the technical fields of medical photography and graphic arts photography, therefore, attention is being given again to methods of forming images by heat-development processing.
In the case of a photosensitive material to be exposed to near infrared, infrared or red lasers, dyes capable of producing sufficient e effects in preventing irradiation and halation at the wavelengths of exposure light are ordinarily incorporated into the photosensitive material for the purpose of forming images of high sharpness. In the heat-development processing, however, the removal of the dyes is difficult, so that the removal or decoloration of the dyes becomes a big problem.
The methods of decoloring dyes by heating in the heat development processing are proposed. For instance, the method of decoloring polymethine dyes of specific structure by heating is disclosed in U.S. Pat. No. 5,135,842. And the methods of decoloring polymethine dyes by heating in the presence of carbanion-producing agents are disclosed in U.S. Pat. Nos. 5,314,795, 5,324,627 and 5,384,237.
As to photosensitive materials for exposure to near infrared or infrared lasers, the photosensitive materials containing substantially no decoloring mechanisms but using dyes having the absorption maximum in the near infrared region, small half-value width and little absorption in the visible ranges are proposed, e.g., in JP-A-9-146220 and JP-A-11-228698.
As to photosensitive materials for exposure to red lasers, however, effective means are limited to adopting complex decoloring reaction mechanisms. The problem arising when the decoloring mechanisms are adopted is that dyes cannot be decolored to a sufficient extent or, on the contrary, the dyes are insufficient in stability, so that they are decolored during storage of the heat-developable photosensitive materials. In the case of using polymethine dyes, there comes up a further problem that the decomposition products of dyes remaining after decoloration have a little absorption of light and make color stains on images (particularly in highlight areas). In addition, there arises a problem that the dyes recover their colors after heat development (especially by contact with an acid), or there occurs a case where the by-products remaining after the complex reaction mechanism cause deterioration in easiness of handling of the photosensitive material after processing.
As to the case of employing no decoloring reaction mechanism, on the other hand, limitation to applications where visible images are not viewed or the method of enjoying visible images by peeling the antihalation layer away (though a waste material is multiplied) is disclosed, e.g., in JP-A-7-13294. In addition, the method of using additional coloring dyes other than dyes for antihalation is disclosed, e.g., in JP-A-2000-29164, but the highlight areas reproduced thereby do not attain to the same level as those in the images having undergone wet processing. Under the circumstances, methods capable of reaching practical utilization are not found yet. Thus, it has been desired to develop the arts of avoiding the need to employ any decoloring mechanism in photosensitive materials for exposure to red lasers.